1) Field of the Invention
The present invention relates to an inter-unit digital signal transmitting method, a digital signal transmitter and receiver equipment, a digital signal transmitter, and a digital signal receiver.
2) Description of the Related Art
In recent years, with an increasing number of mobile telecommunications base stations, inter-cellular telecommunications between exchanges have been frequently utilized. In the radio telecommunications such as mobile telecommunications, as shown in FIG. 14, radio terminals are linked to the exchange 24 (EX) via the radio base station (BS) 23 covering a radio terminal area. Optical fiber cable telecommunications or radio telecommunications have been chiefly utilized to link the radio base station (BS) 23 to the exchange 24.
However, an increasing number of radio base stations 23 causes expensive work which requires laying optical fiber cables to link to new exchanges 24. Particularly, it is costly to network new optical fiber cables in urban areas. For that reason, it has been increasingly worthwhile to use wireless (radio) communications, instead of optical fiber cables, for telecommunications between the radio base station 23 and the exchange 24, because of the advantage of installation easiness and economy.
On the other hand, in order to realize a telecommunication between the radio base station 23 and the exchange 24, they each must include the transmitter unit and the receiver unit to exchange radio data. In this case, the transmitter in each radio base station modulates an electrical signal in the baseband to a signal in an intermediate frequency (IF) band, subjects the IF band signal to a RF band frequency conversion process, and then transmits the outcome.
Various limitations sometimes require that the radio base station 23 or exchange 24 be divided into a first unit for handling digital signals and a second unit for performing a frequency conversion process, each being installed at a different place. It has long been desired to perform an efficient data transmission with no signal distortion between the separated units.
In response to such a desire, digital signal transmitter/receiver equipment has been used as shown in FIGS. 10 to 12. The digital signal transmitter/receiver equipment is constituted of the first transmitter/receiver unit acting as the first unit and the second transmitter/receiver unit acting as the second unit. FIG. 10 is a schematic diagram showing a device that performs a baseband transmission between first and second units. FIG. 11 is a schematic diagram showing a device that performs an IF band transmission using two local sources. FIG. 12 is a schematic diagram showing a device that performs an IF band transmission using one local source.
Explanation will be made below for the devices described above.
First, the digital signal transmitter/receiver equipment shown in FIG. 10 will be explained. Numeral 1 represents the first transmitter/receiver unit that handles digital signals and 2 represents the second transmitter/receiver unit that subjects a digital signal to a modulation/demodulation process and a frequency conversion process.
The first transmitter/receiver unit 1 is connected to the second transmitter/receiver unit 2 via three cables: the communication lines 8a and 8b and the power source cable 8c. The digital signal is subjected to the baseband transmission via the transmission lines 8a and 8b.
The first transmitter/receiver unit 1 is chiefly installed indoors. The first transmitter/receiver unit 1 is constituted of the bipolar/unipolar converting means (hereinafter referred to B/U converting means) 5, the unipolar/bipolar converting means (hereinafter referred to U/B converting means) 9, the CMI encoding means (CMI COD) 6A, and the CMI decoding means (CMI DCOD) 7B.
The second transmitter/receiver unit 2 is chiefly installed outdoors. The second transmitter/receiver unit 2 is constituted of the transmitting unit including the CMI decoding means 7A, the main modulation unit (MOD) 10, the up-converter 11, the high-power amplifier 16 and the bandpass filter 17; the receiving unit including the bandpass filter 17, the low-noise amplifier 18, the down-converter 13, the main demodulation unit (DEM) 14, and the CMI encoding means 6B; and the circulator 12; the antenna unit 15; and the local oscillator 19.
At the signal transmission time, when the transmitter/receiver unit 1 receives digital signal data in the baseband, the B/U converting means 5 converts the digital signal data from a bipolar signal to an unipolar signal while the CMI encoding means 6A subjects the unipolar signal to a code mark inversion (CMI) encoding process.
The second transmitter/receiver unit 2 receives the encoded signal at the baseband frequency via the sending transmission line 8a. In the second transmitter/receiver unit 2, the CMI decoding means 7A decodes first the input signal. The clock extracting unit 7a arranged in the CMI decoding means 7A synchronizes with the CMI coded signal from the CMI encoding means 6A to time the CMI decoding and the modulation in the main modulation unit 10.
Thereafter, the main modulation unit (MOD) 10 inputs the data signal to modulate it to a signal in the intermediate frequency (IF) signal band. The up-converter 11 receives the local signal from the local oscillator 19 to convert the IF frequency band data signal into a RF frequency band signal. The resultant signal is amplified by the high-power amplifier 16 and then transmitted from the antenna unit 15 by way of the bandpass filter 17 and the circulator 12.
At the signal receiving time, the signal is transmitted along the reverse path to that at the transmission time. That is, the signal received with the antenna unit 15 is input to the down-converter 13 by way of the circulator 12, the bandpath 5, filter 17 and the low-noise amplifier 18.
Next, the down-converter 13 converts the data signal from the RF frequency band to the IF frequency band. Then, the main demodulation unit (DEM) 14 demodulates the IF frequency band signal into the baseband signal and then the CMI encoding means 6B subjects the resultant signal to the CMI encoding process.
The resultant signal in a baseband frequency band is input to the first transmitter/receiver unit 1 via the receiving transmission line 8b. In the first transmitter/receiver unit 1, the CMI decoding means 7B decodes the signal and the U/B to converting means 9 converts the resultant signal into a bipolar signal. The clock extracting unit 7b is similar to the clock unit 7a arranged together with the CMI decoding means 7A.
Next the digital signal transmitter/receiver unit 2 shown in FIG. 11 will be explained briefly. Like the configuration shown in FIG. 10, numeral 1 represents the first transmitter/receiver unit that handles digital signals and 2 represents the second transmitter/receiver unit that subjects the digital signal to a modulation/demodulation process as well as a frequency conversion process.
The first transmitter/receiver unit 1 is connected to the second transmitter/receiver unit 2 via a pair of cables, or the transmission lines 8a and 8b, to transmit the digital signal in the IF frequency band via the same lines 8a and 8b.
That is, at a signal transmission time, the main modulation unit 10 in the first transmitter/receiver unit 1 modulates the digital signal data of the baseband to a signal in the IF frequency band. The resultant signal is transmitted to the second transmitter/receiver unit 2 via the transmission line 8a. The first transmitter/receiver unit 1 also includes the capacitors 20A-1, 20A-2, and coils 21A-1, 21A-2, 21A-1, and the second transmitter/receiver unit 2 also includes the capacitors 20B-1, 20B-2 and coils 21B-1, 21B-2, A DC power source is connected to one ends of the coils 21A- 1, 21A-2, 21B- 1, and 21B-2.
In the second transmitter/receiver 2, the amplifier 25A amplifies an input signal in the IF frequency band. Then, in response to the local signal from the local oscillator 19A, the up-converter 11 converts the data signal from the IF frequency band to the RF frequency band. The resultant signal is transmitted from the antenna unit 15 via the bandpass filter 17 and the circulator 12.
The signal received by the antenna unit 15 is inputted to the down-converter 13 via the circulator 12, the bandpass filter 17, and the low-noise amplifier 18.
In response to a local signal from the local oscillator 19B of which the oscillation frequency is different from that of the local oscillator 19A, the down-converter 13 frequency-converts the data signal from the RF signal band to the IF frequency band and then the amplifier 25B amplifies the converted signal. The resultant signal is transmitted to the main demodulation unit 14 via the transmission line 8b to modulate the resultant signal into a baseband signal.
In the configuration shown in FIG. 1, since the main modulation unit 10 and the main demodulation unit 14 are arranged on the side of the first transmitter/receiver unit 1, the attenuation of digital data due to a cable arranged between the transmitter/receiver units 1 and 2 can be relatively reduced by setting the IF frequency to a small value. However, two cables are needed between the transmitter/receiver units 1 and 2.
Next, the digital signal transmitter/receiver equipment will be explained with reference to FIG. 12. In this configuration, a signal cable (transmission line 8) is connected between the first transmitter/receiver unit 1 and the second transmitter/receiver unit 2. The digital signal is subjected to an IF transmission via the transmission line 8 shared for transmission and reception.
That is, in the configuration shown in FIG. 12, at the signal transmission time, the main modulation unit (MOD) 10 receives the digital signal data in the baseband input to the first transmitter/receiver unit 1 to modulate it to an IF frequency band signal. The modulated signal is transmitted to the second transmitter/receiver unit 2 via the hybrid circuit (composite and branch filter) 22A, 22B and the transmission line 8.
The transmission signal inputted to the second transmitter/receiver unit 2 is branched by the hybrid circuit 22B and then amplified by the amplifier 25A. Moreover, in response to a local signal from the local oscillator 19, the up-converter 11 converts the data signal in the IF frequency band into that in the RF frequency band.
The converted signal is amplified by the high-power amplifier 16 and the resultant signal is then transmitted to the antenna 15 via the bandpass filter 17 and the circulator 12.
At the signal receiving time, the signal is transmitted along the reverse path to that in the transmission time. That is, the signal received by the antenna unit 15 is inputted to the down-converter 13 via the circulator 12, the bandpass filter 17 and the low-noise amplifier 18.
The down-converter 13 converts the data signal in the RF frequency signal into a signal in the IF frequency band in accordance with the local signal from the local oscillator 19 shared as one for the transmitter. Then, the amplifier 25B amplifies the converted signal. Then, the hybrid circuit 22B combines the signal from the amplifier 25B with the signal from the amplifier 25A and then transmits the appropriate digital signal to the first transmitter/receiver unit 1 via the transmission line 8. In the first transmitter/receiver unit 1, the hybrid circuit 22A branches the received signal and the main demodulation unit (DEM) 14 demodulates the resultant signal in the IF frequency band into a signal in the baseband.
The configuration including the local oscillator 19 shared for transmission and reception allows the single cable (transmission line 8) to connect the first transmitter/receiver unit 1 to the second transmitter/receiver unit 2. However, as shown in FIG. 13, the IF frequency of the transmission system or the receiving system is fairly high. This causes a relatively large amount of signal attenuation due to cable losses.
As shown in FIG. 15, the radio base stations 23 are sometimes installed on general buildings (for example, office buildings) in an urban area. If a suitable space cannot be found, the indoor equipment (or the first unit) may be installed in a basement of the building and the outdoor equipment (or the second unit) may be installed on the roof thereof, as shown in FIG. 15. In this case, it may be required to lay very long cables between the indoor equipment and the outdoor equipment.
In the consideration of the installation easiness and the cable laying cost, it is most desirable to select an arrangement in which a signal cable is laid between the indoor equipment and the outdoor equipment, as shown in FIG. 12.
However, the problem arises that this configuration, where a single coaxial cable connects one equipment to another equipment, limits the coaxial cable laying to the length to which the device within the equipment can compensate for cable attenuation.
In other words, in order to lay a longer cable between the indoor equipment and the outdoor equipment without increased cable attenuation, the configuration requires using a cable with a thick inner conductor (with less signal attenuation) or an internal device with a large compensation characteristic for attenuation.
However, thickening an internal conductor in the cable results in a degraded flexibility of cabling as well as an equipment connection difficulty. Moreover, increasing the compensation characteristic of the internal device causes an increased manufacturing cost and bulky equipment.